Resistance furnace with tubular heating element

ABSTRACT

A resistance furnace has a tubular heating element with a vertically-oriented longitudinal axis. The heating element has a shell surface defined by an upper side and a lower side and surrounding a furnace chamber. The heating element is connected to at least two supply terminals by which heating current is introduced at power supply points into the heating element. The supply terminals include a surrounding upper annular collar adjacent the upper side of the shell surface, and also a surrounding lower annular collar adjacent the lower side of the shell surface.

FIELD OF THE INVENTION

The present invention relates to a resistance furnace comprising atubular heating element having a vertically oriented longitudinal axis,the element comprising a shell surface defined by an upper side and alower side and surrounding a furnace chamber, and the element beingconnected to supply terminals by means of which heating current isintroduced into the heating element at power supply points.

BACKGROUND OF THE INVENTION

High temperatures can be achieved with the help of resistance furnaces.Electrical current flows through an ohmic resistor designed as a heatconductor, the electrical power being mainly converted into heat. Metal,such as molybdenum, tantalum, and platinum, ceramics, SiC, ormodifications of carbon, such as coal, graphite or vitreous carbon(pyrolytically produced carbon) are suited as a material for the heatconductor. Heat conductors of graphite are characterized by their hightemperature resistance, simple shape and low price.

Resistance furnaces are e.g. used for melting semiconductor material, orfor heating rod-like or tubular start cylinders to elongate tubes, rodsor optical fibers therefrom. Apart from special cases in which apurposefully inhomogeneous heating capacity is desired (e.g. duringdrawing of cylinders having a polygonal cross-section), the mainemphasis is normally laid on a uniform heating of the heating material.

The local heating capacity is directly proportional to the currentdensity, the latter being defined by the current flow and thecross-sectional area and the specific resistance of the material of theheat conductor. The specific resistance, in turn, depends on the localtemperature. The relatively low conductivity of the heat conductormaterials and the accompanying voltage drop make it difficult to produceand maintain a homogeneous temperature profile.

U.S. Pat. No. 4,703,556 therefore suggests a furnace having a heatingtube of graphite, in which a plurality of axially extending longitudinalslots are distributed over the circumference of the heating tube andextend in alternating fashion from above and from below over almost thewhole heating tube height. Thus, electricity flows through the remainingwebs of the heating tube in meander-like fashion. This results in ahomogenization of the temperature curve in vertical direction.

A further homogenization of the current density within the heating tubeis accomplished by the measure that two graphite connection pieces areprovided for the supply of heating current, said pieces being screwed inthe area of the bottom side of the heating tube to opposite places andbeing fed via separate transformers. The voltage drop can becounteracted by the second power supply point over the entire length ofthe heating tube webs, resulting in an improved vertical and horizontalhomogeneity of the current and temperature distribution.

It is however very complicated to produce the known heating tube. Due toits filigree shape it is prone to mechanical damage and must thereforebe replaced frequently. Moreover, due to scaling and oxidation in theregion of the supply terminals there might be a deterioration of theelectrical contact and an undefined power supply and thus an irregularheating operation and temperature distribution over the heating tube.This risk is enhanced by the fact that the supply terminals are locateddirectly next to the zone with the maximum temperature load.

In high-performance resistance furnaces (above about 100 kW),temperature sinks are particularly noticed and make it more and moredifficult to adjust and maintain a homogeneous temperature distribution.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an electricresistance furnace, in particular a high-performance resistance furnacewhich is characterized by small manufacturing and maintenance effortsand by a high operational reliability, and which permits thereproducible adjustment of an axially and radially homogeneoustemperature profile within the furnace.

Starting from the above-mentioned resistance furnace, this object isachieved according to the invention in that the supply terminalscomprise a surrounding upper annular collar of metal in the region ofthe upper side and a surrounding lower annular collar of metal in theregion of the lower side.

Heating current is introduced into the heating element via the annularcollars. To this end one annular collar is provided at the upper frontend and one annular collar at the lower front end of the heatingelement. The annular collars are in contact with the respective frontside of the tubular heating elements with which the inner jacket and/orthe outer jacket are in contact, the contact portion being preferablydesigned in planar fashion, in the simplest case as an annular contactsurface between annular collar and heating element. The annular collarsare made from one piece or are composed of individual members.

For the supply of the heating current into the respective annularcollar, said collars are each provided with at least one currentterminal. The at least one current terminal of the annular collars willalso be designated in the following as an “electrode terminal”.

The component of the resistance furnace supplied with heating current ishere understood as the heating element. It may be made integral or maybe composed of a plurality of members.

The annular collars are made from a metal of high electricalconductivity, so that a horizontal voltage drop that is as small aspossible is achieved when viewed over the circumference thereof. Sincethe annular collars are provided both on the upper side and the lowerside of the heating element and current is thereby supplied to theheating element from two sides, there is also a reduced vertical voltagedrop over the height of the heating element in comparison with a currentsupply from only one side. This results in a homogeneity of the currentand temperature distribution that is improved on the whole.

Especially with respect to a homogeneous horizontal distribution of thecurrent power density within the heating element, it has been found tobe useful when each annular collar comprises an electrode terminal forthe supply of heating current, the electrode terminal of the upperannular collar, when viewed circumferentially, being offset relative tothe electrode terminal of the lower annular collar.

In the case of only one electrode terminal, maximum power density isachieved within the respective annular collar always in the area of theelectrode terminal and the lowest power density is normally found at theside of the annular collar that is furthest away. To achieve acompensation, it has turned out to be of advantage when each annularcollar is provided with at least two electrode terminals that are evenlydistributed around the annular collar in circumferential direction.Advantageously, the electrode terminals are evenly distributed over thecircumference of the annular collar.

Likewise, the maximum power density is also obtained within the contactregion between annular collar and heating element, normally in thedirect vicinity of an electrode terminal (although the annular collar isprovided with a plurality of electrode terminals). For the purpose ofcompensation, regions of a lower electrical conductivity are formed in aparticularly preferred embodiment of the resistance furnace in theannular collar for branching the supplied heating current into at leastfour, preferably into at least eight, current paths which lead tofavored power supply points that are uniformly distributed over thecircumference of the heating element.

“Current path” in this context means the distance covered by the heatingcurrent from the electrode terminal to the contact region on the heatingelement. The shorter a current path is, the higher is the currentdensity in the contact region. Desired is, however, a current densitythat is as uniform as possible on the surrounding contact surfacebetween annular collar and heating element. According to the inventionthis is achieved by the measure that the heating current fed into theannular collar is branched once or several times, so that at least fourcurrent paths are formed. The branching is accomplished by regions whichhave a lower electrical conductivity than the remaining material of theannular collar. Due to the lower conductivity (which may also be zero),the current flows around said regions substantially such that thecurrent flow can be divided and force-guided. For instance, it ispossible with the help of radially extending longitudinal slots in theannular collar to interrupt the “shortest path” from the respectiveelectrode terminal to the contact region. Ideally, the four or morecurrent paths have the same length. The area in which the respectivecurrent path ends is here designated as the “most favored power supplypoint”. At any rate, the high electrical conductivity of the annularcollar effects an approximately uniform horizontal distribution of theheating current around the regions of lower electrical conductivity. Theplanar contact region towards the heating element that in considerationof the forced guidance over the current paths has a relatively shortdistance from the electrode terminal and thus a still increased currentdensity despite an approximately uniform current density distribution isdesignated as the “most favored power supply point”. An embodiment withfour favored power supply points will also be designated hereinafter asa “4-point supply”, and an embodiment with eight favored power supplypoints accordingly as an “8-point supply”.

Thanks to a uniform distribution of favored power supply points aroundthe circumference of the heating element, a uniform radial currentdensity distribution is achieved around the heating element, whichadditionally improves the homogeneity of the current and temperaturedistribution. This measure is in particular positively felt at highheating capacities (above 100 kW).

One embodiment of the resistance furnace of the invention has turned outto be particularly useful wherein, when viewed circumferentially, thefavored power supply points of the upper annular collar are offsetrelative to the power supply points of the lower annular collar. Onaccount of the offset of the lower power supply points to the upperpower supply points, the power density curve can be further homogenizedvertically (axially), resulting in an improved temperature homogeneity.The offset between the power supply points of the upper and lowerannular collar is normally such that a rotational symmetry is obtainedrelative to the longitudinal axis of the heating tube.

Advantageously, the annular collars are provided with a connection conematching a conical connection region of the heating element. Thanks tothe cone, annular collar and heating tube are self-centered, resultingin a firm fixation so that changes in the contact resistance are avoidedduring use of the furnace. The annular collar is made integral or iscomposed of a plurality of components. In particular, the connectioncone may be manufactured as a separate component. A more advantageousforce distribution is obtained when the connection cone is designed asan inner cone and the connection region of the heating element as anouter cone. The annular collars are here seated with the connection coneon the outer cone of the heating element.

Preferably, the annular collars are provided with a first surroundingcooling channel having a first coolant inlet. Due to cooling thetemperature of the annular collars and thus the specific ohmicresistance are kept constant locally and in time.

In this respect it has turned out to be of particular advantage when theannular collars provided next to the first cooling channel and spatiallyseparated therefrom comprise a second surrounding cooling channel with asecond coolant inlet, the second inlet, when viewed in circumferentialdirection, being arranged at the side of the annular collar that isopposite the first inlet. A cooling operation in countercurrent flow ispossible by means of the two cooling channels, which contributes to afurther homogenization of the horizontal temperature distribution viathe annular collars.

In an annular collar which, as has been explained above, is providedwith a connection cone, the cooling channel or cooling channels arepreferably formed in the area of the connection cone. Said area need notbe electrically isolated from the area of the annular collar in whichthe heating current is branched into the current paths.

Due to their high electrical conductivity, annular collars of copper orof a copper alloy are preferably used. The high conductivity effects alow voltage drop, which facilitates the observance of a homogeneousdistribution of the current and power density within the annular collarsand thus also in the contact region towards the heating tube.

Preference is given to an embodiment of the resistance furnace accordingto the invention, wherein the heating element comprises a heating tubeof a smaller wall thickness which is extended at both sides by means ofat least one contact tube following at the front side and being of agreater wall thickness, each of the annular collars resting on thecontact tube.

In this preferred embodiment of the resistance furnace according to theinvention, a contact tube is provided between the heating tube properand the annular collars. The highest temperature is observed in the areaof the thin-walled heating tube whereas the contact tube is less heateddue to its greater wall thickness and, consequently, the annular collarsare also subjected to a smaller temperature load. The risk of a changein the contact resistance due to scaling or oxidation is therebyreduced. The heating tube and the contact tubes next thereto arenormally designed as separate components. The contact tubes may be madeintegral or be composed of several individual parts. It is hereessential that the temperature in the area of the contact tubes issmaller than the temperature in the area of the heating tube. To thisend, and instead of a greater wall thickness, the contact tubes may alsobe made from a material of a lower specific resistance than the heatingtube. The design of the heating element with contact tubes makes theresistance furnace of the invention well suited for use with highheating capacities.

A further improvement with respect to the homogeneity of the temperaturedistribution is achieved when a clamping means is provided thatcomprises a plurality of clamping elements by means of which the contacttubes, the heating tube and the annular collars are axially clampedrelative to one another. The clamping means ensures a constantelectrical contact between the respective members and minimizes localchanges in the contact resistance. Preferably, at least four tensionrods that are uniformly distributed over the circumference of theheating element are provided as clamping elements. The rotationallysymmetrical arrangement of the tension rods (around the longitudinalaxis of the heating element) contributes to a symmetrical currentdensity and temperature distribution in the heating element. The tensionrods comprise, for instance, a spring having an exact spring constant,through which a uniform bias can be set in a reproducible and checkablemanner.

It has been found to be of particular advantage when a means for gasflushing is provided for introducing a gas flow into the heatingelement, the means comprising a gas inlet which is branched in at leastone branch stage, preferably in at least three branch stages, into aplurality of secondary lines of equal pressure loss, the secondary linesterminating in a plurality of gas outlets which are uniformlydistributed over an envelope circle and directed into the furnacechamber.

The gas inlet is branched into at least two secondary lines which, inturn, terminate in a plurality of gas outlets which are uniformlydistributed over an envelope circle and directed into the furnacechamber. The gas outlets form a substantially annular cross-sectiontogether with a gas shower. The gas flow passes through the heatingelement in axial direction (in the direction of the verticallongitudinal axis). To achieve a gas flow that is as homogeneous andlaminar as possible, the gas pressure applied to the gas outlets shouldbe the same, if possible. This is accomplished when the pressure losseffected by the secondary lines is as identical as possible. Thepressure loss is defined by the flow cross-section and by the path ofthe gas between gas inlet and opening of the respective secondary line.The flow cross-sections and the paths of the secondary lines are thusideally identical. The greater the number of the branches is, the morehomogeneous can the annular gas flushing operation be realized. However,the manufacturing efforts also increase with the number of branches. Anoptimum compromise is achieved in the case of three branches. Theabove-described gas shower is just as well suited for use in combinationwith a resistance furnace of the invention as for use in combinationwith an induction furnace.

In this respect it has been found to be of particular advantage when thegas outlets are formed in a flush ring which is arranged above the upperannular collar and is composed of at least two, preferably at leastfour, separate circular segments, the secondary lines each terminatingin one of the circular segments. Four circular segments are obtainedwhen the gas inlet is branched twice into first two and then foursecondary lines. The secondary lines preferably terminate mid-way in arespective one of the circular segments, resulting in a furtherbranching of the gas flow. The segments forming the flush ring arepositioned on the above-mentioned envelope circle. The gas pressureapplied in front of the gas outlets within the circular segments issubstantially identical; because of the branches, this is due to thefact that the flush gas covers approximately the same distance from thegas inlet to the gas outlets. A preferred development of the resistancefurnace according to the invention is characterized in that the heatingelement is surrounded by a protective jacket of a controllabletemperature whose outer wall has detachably secured thereto coolingplates within which a cooling liquid is flowing.

Cooling plates are detachably secured to the outer wall of theprotective jacket, a cooling liquid flowing within the cooling plates.The cooling plates are connected to a separate supply means for thecooling liquid, or it is also possible to connect a plurality of coolingplates in series or in parallel for supplying cooling liquid. Thetemperature distribution within the heating element can also be variedby means of the cooling plates, a locally different temperature controlof the protective jacket being also possible. The detachable connectionof the cooling plates with the protective jacket guarantees a simpleexchange.

It has been found to be of advantage when the interior of the heatingelement is sealed to the outside by means of quartz wool. Quartz wool ischaracterized by a high temperature resistance. In case the resistancefurnace is used for heating quartz glass bodies, quartz wool is amaterial of the same type. Quartz wool may be cleaned beforehand, e.g.by chemical etching or hot chlorination. A further improvement will beachieved when the quartz wool consists of synthetically produced SiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be explained with reference to an embodiment anda drawing. The drawing is a schematic illustration showing in detail in:

FIG. 1 an embodiment of a resistance furnace according to the inventionin a side view;

FIG. 2 an embodiment of an annular collar for supplying heating currentinto a heating tube by means of an 8-point supply, in a top view; and

FIG. 3 an embodiment of a flush ring for producing a laminar flush gasflow in the shape of an annular gap, in a top view.

DETAILED DESCRIPTION

The resistance furnace according to the invention as shown in FIG. 1 isused for heating and elongating a cylinder 1 of quartz glass. Thisfurnace is a high-performance furnace having a maximum heating capacityof 700 kW. The resistance furnace comprises a heating tube 2 of graphitewith a vertically oriented longitudinal axis 3, the heating tubeenclosing a heating chamber 4.

Contact tubes 5; 6 are positioned on the heating tube 2 at both sides atthe front. The contact tubes 5; 6 are also made from graphite and have agreater wall thickness than the heating tube 2.

Heating current is introduced into the heating tube 2 via annularcollars 7; 8 consisting of a copper alloy CuCrZr, an upper annularcollar 7 resting on the upper side 11 of the upper contact tube 5 and alower annular collar 8 on the lower side 12 of the lower contact tube 6.The annular collars 7; 8 comprise outwardly projecting flanges 31 whichhave provided therein through holes each extending radially over part ofthe circumference of the annular collars 7; 8 and used for branching theheating current supplied to the annular collars 7; 8 into eight currentpaths of approximately the same length, resulting in an 8-point supply.The shape and function of the annular collars 7; 8 will be described inthis respect in more detail further below with reference to FIG. 2.

Each of the annular collars 7; 8 is provided with two surroundingcooling channels 9; 10 which extend in neighboring relationship witheach other and by which water is guided in countercurrent flow forcooling the annular collars 7; 8. With the help of this water cooling,the temperature of the annular collars 7; 8—and thus the specificresistance—is kept constant locally and in time. The cooling water inlet27 of the one cooling channel 9 is opposite the cooling water inlet 28of the other cooling channel 10. With the help of the two coolingchannels 9; 10 it is possible to cool the annular collars 7; 8 incountercurrent flow, which contributes to a further homogenization ofthe horizontal temperature distribution via the annular collars 7; 8.

For fixing the annular collars 7; 8 onto the contact tubes 5; 6, upperside 11 of the upper contact tube 5 and lower side 12 of the lowercontact tube 6 are provided with a surrounding outer cone which matchesan inner cone of the respectively assigned annular collar 7; 8.

Heating tube 2, contact tubes 5; 6 and annular collars 7; 8 are clampedrelative to one another by means of a clamping device 13. The clampingdevice 13 comprises four tension rods 14 that are evenly distributedover the circumference of the heating tube 2 and connected to a bottomplate 30. Said tension rods are each provided with an adjustableclamping spring 15 of a defined spring constant by means of which saidfurnace members (2; 5; 6; 7; 8) are pressed via spacers 16 at apredetermined and measurable pressing force onto one another, therotationally symmetrical distribution of the tension rods 14 alsoeffecting a symmetry of the compressive stress distribution, which hasan advantageous effect on the homogeneous distribution of currentdensity and temperature within the heating tube.

For the introduction of a flush gas flow (directional arrows 17) intothe furnace chamber 4, a flush gas ring 18 is arranged above the upperannular collar 7 and is provided on its circumference with gas outletsdirected into the furnace chamber 4. The flush gas ring 18 consists offour separate circular segments, a secondary line 20 branched from a gasinlet 19 terminating in each of said segments. Details regarding theconstruction and function of the flush gas ring 18 will be describedfurther below with reference to FIG. 3.

For the purpose of heat shielding the heating tube 2 and the contacttubes 5; 6 are surrounded by a furnace jacket 21 of a controllabletemperature with cooling plates 29 being screwed in segments onto theouter jacket thereof. A cooling liquid flows through the cooling plates29, the cooling circuits of the cooling plates 29 being separated fromone another, so that the furnace jacket may have locally differenttemperatures. To prevent the withdrawal of heat from the furnace chamber4, a gap is provided between the cooling plates 29 and the furnacejacket 21, so that the cooling plates 29 do not directly rest on thefurnace jacket 21.

The furnace jacket 21 is provided in the center with a through hole 22through which the temperature of the heating tube 2 is measured by meansof an optical waveguide 23. With the help of the optical waveguide 23,the temperature measurement signal is transmitted to a measuring device.The measuring device can thus be so far away from the heating tube 2that a cooling operation is not needed for the protection of themeasuring device.

The gap between the secondary lines 20 of the flush ring 18 and thequartz glass cylinder 1 is sealed by means of purified quartz wool 26.

FIG. 2 is a schematic illustration showing the upper annular collar 7 ina top view. The annular collar 7 consists of a substantially ring-likecopper disk which is provided with two electrode terminals 71; 72(terminal tabs) which are opposite each other on the outer circumferenceand via which the heating current is fed into the annular collar 7. Theelectrode terminals 71; 72 are fed by a joint power source (not shown inthe figure). The annular collar 7 is provided with a plurality ofthrough holes 73 which extend radially around part of the circumferenceand are distributed in symmetry around the central axis 74(corresponding to the longitudinal axis 3). The through holes 73 arehere formed in the area of the flange 31 and arranged such that theheating current supplied by the electrode terminals 71; 72 is branchedinto eight current paths of equal length, symbolized by directionalarrows 75, and thereby guided to “favored supply points” 76 in the areaof the inner hole 77 with which the annular collar 7 rests on thecontact tube 5 (see FIG. 1). Thanks to the two electrode terminals 71,72 and the distribution and size of the through holes 73, the heatingcurrent is distributed in the annular collar 7 according to FIG. 2 overa total of eight favored supply points 76 (“8-point supply”). Incomparison with an embodiment of the annular collar with only one“favored supply point”, this results in a more uniform current densitydistribution in the area of the inner hole 77 and thus to a homogeneoushorizontal distribution of the current density in the area of thecontact tube 5 and thus also of the heating tube 2.

A further homogenization of the current density distribution in theheating tube 2 is achieved by the measure that the electrode terminals71; 72 of the upper annular collar 7 are offset by 90° relative to theelectrode terminals 78; 79 of the lower annular collar 8 (see FIG. 1),as is schematically outlined in FIG. 2. The lower annular collar 8 isalso optimized for an “8-point supply” of the heating current, so thatthe offset arrangement of the annular collars 7; 8 also results in anoffset of the “favored power supply points” by 90° and in rotationalsymmetry with respect to the central axis 74. This results in a uniformvertical current density distribution within the heating tube 2, whichadditionally contributes to an improved homogeneity of the current andtemperature distribution.

To avoid the entry of oxygen into the furnace chamber 4, said chamber isflushed with nitrogen during the drawing process. To this end a flushring 18 (FIG. 1) is used, as shown in FIG. 3. The flush ring 18 consistsof interconnected copper tubes forming a gas inlet nozzle 81 branched intwo branch stages 82; 83 into a total of four secondary lines 20. Eachof the secondary lines 20 centrally terminates—under formation of athird branch stage 85—in an annular segment 89 which is provided with aplurality of gas outlets 86. The gas outlets 85 of the annular segments89 are positioned on an envelope circle smaller than the inner diameterof the heating tube 2. The gas outlets 85 jointly form a gas shower of asubstantially annular cross-section. The nitrogen flow, starting fromthe gas outlets 86, passes through the heating tube 2 towards thelongitudinal axis 3 from the top to the bottom. To obtain a nitrogenflow that is as homogeneous and laminar as possible, a gas pressure thatis the same, if possible, is desired at the gas outlets 8. This isaccomplished in that all secondary lines 20 have the same length and thesame inner diameter and that the distance between the first branch stage82 and the opening 87 in the respective annular segment 89 is the samefor all secondary lines 20.

1. A resistance furnace comprising: a tubular heating element having avertically oriented longitudinal axis, said heating element having ashell surface defined by an upper side and a lower side and surroundinga furnace chamber, and being connected to at least two supply terminalsby means of which heating current is introduced at power supply pointsinto said heating element, wherein said supply terminals comprise asurrounding upper annular collar adjacent said upper side and asurrounding lower annular collar adjacent said lower side, whereinregions of lower electrical conductivity are formed in said annularcollars for branching the supplied heating current into at least fourcurrent paths leading to favored power supply points uniformlydistributed over a circumference of said heating element, and whereinsaid power supply points of said upper annular collar are offsetcircumferentially relative to said power supply points of said lowerannular collar.
 2. A resistance furnace comprising: a tubular heatingelement having a vertically oriented longitudinal axis, said heatingelement having a shell surface defined by an upper side and a lower sideand surrounding a furnace chamber, and being connected to at least twosupply terminals by means of which heating current is introduced atpower supply points into said heating element, wherein said supplyterminals comprise a surrounding upper annular collar adjacent saidupper side and a surrounding lower annular collar adjacent said lowerside, and wherein each of said annular collars comprises an electrodeterminal feeding said heating current, and wherein said electrodeterminal of said upper annular collar is offset circumferentiallyrelative to said electrode terminal of said lower annular collar.
 3. Theresistance furnace according to claim 2, wherein each of said annularcollars is provided with at least two electrode terminals uniformlydistributed circumferentially around said annular collar.
 4. Theresistance furnace according to claim 2 wherein regions of lowerelectrical conductivity are formed in said annular collars for branchingthe supplied heating current into at least four current paths leading tofavored power supply points uniformly distributed over a circumferenceof said heating element.
 5. The resistance furnace according to claim 2wherein said heating element comprises a heating tube having a wallthickness, said heating tube being extended at both sides by means of atleast one contact tube having a front side following a front side of theheating tube and having a greater wall thickness, each of said annularcollars being supported on said contact tube.
 6. The resistance furnaceaccording to claim 5, wherein there is provided a clamping means whichcomprises a plurality of clamping elements by means of which saidcontact tubes, said heating tube and said annular collars are axiallyclamped relative to one another.
 7. The resistance furnace according toclaim 6, wherein at least four tension rods which are uniformlydistributed over a circumference of said heating element are provided asclamping elements.
 8. The resistance furnace according to claim 2wherein regions of lower electrical conductivity are formed in saidannular collar for branching the supplied heating current into at leasteight current paths leading to favored power supply points uniformlydistributed over a circumference of said heating element.
 9. Theresistance furnace according to claim 2 wherein said annular collarseach comprise a connection cone matching a respective conical connectionregion of said heating element.
 10. The resistance furnace according toclaim 3 wherein said annular collars each comprise a connection conematching a respective conical connection region of said heating element.11. The resistance furnace according to claim 4 wherein said annularcollars each comprise a connection cone matching a respective conicalconnection region of said heating element.
 12. The resistance furnaceaccording to claim 2 wherein said annular collars have a firstsurrounding cooling channel having a first coolant inlet.
 13. Theresistance furnace according to claim 3 wherein said annular collarshave a first surrounding cooling channel having a first coolant inlet.14. The resistance furnace according to claim 9, wherein said connectioncone is an inner cone and the connection region of said heating elementis an outer cone.
 15. The resistance furnace according to claim 12,wherein said annular collars have a second surrounding cooling channeladjacent said first cooling channel and spatially separated therefrom,said second surrounding cooling channel having a second coolant inlet,said second coolant inlet arranged at a side of said annular collarcircumferentially opposite said first inlet.
 16. The resistance furnaceaccording to claim 2 wherein said heating element is surrounded by aprotective jacket of a controllable temperature whose outer wall hasdetachably secured thereto cooling plates within which a cooling liquidis flowing.
 17. A resistance furnace comprising: a tubular heatingelement having a vertically oriented longitudinal axis, said heatingelement having a shell surface defined by an upper side and a lower sideand surrounding a furnace chamber, and being connected to at least twosupply terminals by means of which heating current is introduced atpower supply points into said heating element, wherein said supplyterminals comprise a surrounding upper annular collar adjacent saidupper side and a surrounding lower annular collar adjacent said lowerside, and wherein the heating element has an outside and an interior,and the interior of said heating element is sealed to the outside bymeans of quartz wool.
 18. The resistance furnace according to claim 17,wherein said quartz wool consists of synthetically produced SiO₂.
 19. Aresistance furnace comprising: a tubular heating element having avertically oriented longitudinal axis, said heating element having ashell surface defined by an upper side and a lower side and surroundinga furnace chamber, and being connected to at least two supply terminalsby means of which heating current is introduced at power supply pointsinto said heating element, wherein said supply terminals comprise asurrounding upper annular collar adjacent said upper side and asurrounding lower annular collar adjacent said lower side, and whereingas flushing is provided by a system which comprises a gas inlet whichis branched in at least one branch stage, into a plurality of secondarylines of equal pressure loss, said secondary lines terminating in aplurality of gas outlets which are uniformly distributed over anenvelope circle and directed into said furnace chamber.
 20. Theresistance furnace according to claim 19, wherein said gas outlets areformed in a flush ring which is arranged above said upper annular collarand is composed of at least two separate circular segments, saidsecondary lines each terminating in one of said circular segments. 21.The resistance furnace according to claim 19, wherein said gas outletsare formed in a flush ring which is arranged above said upper annularcollar and is composed of at least four separate circular segments, saidsecondary lines each terminating in one of said circular segments. 22.The resistance furnace according to claim 19 wherein said gas inlet isbranched in at least three branch stages, into said plurality ofsecondary lines of equal pressure loss.
 23. The resistance furnaceaccording to claim 1 wherein said annular collars each comprise aconnection cone matching a respective conical connection region of saidheating element.
 24. The resistance furnace according to claim 1 whereinsaid annular collars have a first surrounding cooling channel having afirst coolant inlet.
 25. The resistance furnace according to claim 2wherein said annular collars consist of copper or a copper alloy. 26.The resistance furnace according to claim 3 wherein said annular collarsconsist of copper or a copper alloy.
 27. The resistance furnaceaccording to claim 4 wherein said annular collars consist of copper or acopper alloy.
 28. The resistance furnace according to claim 1 whereinsaid annular collars consist of copper or a copper alloy.
 29. Theresistance furnace according to claim 5 wherein said heating element issurrounded by a protective jacket of a controllable temperature whoseouter wall has detachably secured thereto cooling plates within which acooling liquid is flowing.
 30. The resistance furnace according to claim6 wherein said heating element is surrounded by a protective jacket of acontrollable temperature whose outer wall has detachably secured theretocooling plates within which a cooling liquid is flowing.
 31. Theresistance furnace according to claim 7 wherein said heating element issurrounded by a protective jacket of a controllable temperature whoseouter wall has detachably secured thereto cooling plates within which acooling liquid is flowing.
 32. The resistance furnace according to claim1 wherein said annular collars have a first surrounding cooling channelhaving a first coolant inlet.
 33. The resistance furnace according toclaim 32, wherein said annular collars have a second surrounding coolingchannel adjacent said first cooling channel and spatially separatedtherefrom, said second surrounding cooling channel having a secondcoolant inlet, said second coolant inlet arranged at a side of saidannular collar circumferentially opposite said first inlet.
 34. Theresistance furnace according to claim 1 wherein said heating elementcomprises a heating tube having a wall thickness, said heating tubebeing extended at both sides by means of at least one contact tubehaving a front side following a front side of the heating tube andhaving a greater wall thickness, each of said annular collars beingsupported on said contact tube.
 35. The resistance furnace according toclaim 34, wherein there is provided a clamping means which comprises aplurality of clamping elements by means of which said contact tubes,said heating tube and said annular collars are axially clamped relativeto one another.
 36. The resistance furnace according to claim 35,wherein at least four tension rods which are uniformly distributed overa circumference of said heating element are provided as clampingelements.
 37. The resistance furnace according to claim 1 wherein saidheating element is surrounded by a protective jacket of a controllabletemperature whose outer wall has detachably secured thereto coolingplates within which a cooling liquid is flowing.
 38. The resistancefurnace according to claim 1 wherein regions of lower electricalconductivity are formed in said annular collar for branching thesupplied heating current into at least eight current paths leading tofavored power supply points uniformly distributed over a circumferenceof said heating element.
 39. The resistance furnace according to claim17 wherein said heating element comprises a heating tube having a wallthickness, said heating tube being extended at both sides by means of atleast one contact tube having a front side following a front side of theheating tube and having a greater wall thickness, each of said annularcollars being supported on said contact tube.
 40. The resistance furnaceaccording to claim 39, wherein there is provided a clamping means whichcomprises a plurality of clamping elements by means of which saidcontact tubes, said heating tube and said annular collars are axiallyclamped relative to one another.
 41. The resistance furnace according toclaim 40, wherein at least four tension rods which are uniformlydistributed over a circumference of said heating element are provided asclamping elements.
 42. A resistance furnace comprising: a tubularheating element having a vertically oriented longitudinal axis, saidheating element having a shell surface defined by an upper side and alower side and surrounding a furnace chamber, and being connected to atleast two supply terminals by means of which heating current isintroduced at power supply points into said heating element, whereinsaid supply terminals comprise a surrounding upper annular collaradjacent said upper side and a surrounding lower annular collar adjacentsaid lower side, and wherein regions of lower electrical conductivityare formed in said annular collars for branching the supplied heatingcurrent into at least four current paths leading to favored power supplypoints uniformly distributed over a circumference of said heatingelement, and wherein said annular collars have a first surroundingcooling channel having a first coolant inlet.